The T-Phonum MKII is a high-performance RIAA phono preamplifier for MM and MC cartridges.
The T-Phonum MKII has been specifically designed in order to get the best performance in the reproduction of vinyl records. A novel two-stage RIAA network, consisting of an active and a passive stage, allows overcoming the main limitations of accuracy, noise and input headroom that typically affects traditional devices.
Through a complex design procedure, an exceptional accuracy in the RIAA frequency response is obtained, ensuring an absolute fidelity in the audio reproduction. In addition, the particular combination of the two-stage RIAA network and the unique topology of the gain stages allows a significant increase of the signal to noise ratio and dynamic respect to conventional circuitry.
|Gain||MM: 40 dB, 46 dB|
MC: 60 dB, 66 dB
|4 user selectable gains|
|Signal to noise ratio||A-weight:|
MM: 92 dB, 87 dB
MC: 73 dB, 67 dB
MM: 88 dB, 83 dB
MC: 69 dB, 63 dB
|Maximum input signal||81 mVRMS (230 mVpp) a 40 dB|
7,8 mVRMS (22,2 mVpp) a 60 dB
|1 kHz, ±12 V power supply|
|Headroom||27 dB||1 kHz, ±12 V power supply|
|THD||~0,002%||Op-amp LT1115, 1 kHz|
|RIAA accuracy||±0,05 dB||Typ., 20 Hz ÷ 20 kHz|
|Input capacitance||0 pF, 100 pF, 150 pF, 220 pF, 250 pF, 320 pF, 370 pF, 470 pF||8 user selectable values|
|Input resistance||47 kΩ, 1 kΩ, 100 Ω, 91 Ω||4 user selectable values|
Gain (for both MM and MC), input capacitance and input resistance are user-selectable through high-quality miniature switches.
Any aspect of this project, from the electrical design to the versatility of the settings, is specifically developed in order to reach high-end musical performance.
Each channel uses two gain stages made by two high-performance audio operational amplifiers. The first gain stage drives the two-stage RIAA network, which is specifically designed for a low impedance in order to reduce the thermal noise. This first stage provides a significant amount of the whole gain.
The second op-amp provides the remaining amplification, that can be adjusted to meet the desired total gain depending on the specific cartridge used. This unique combination of equalization and amplification underlies the high signal to noise ratio and the high input dynamic of T-Phonum MKII.
In order to get the best noise performance, the first op-amp should provide a low input noise. For example, the following types, ordered by their performance, are suitable.
|Op-amp (first stage)||Input noise|
|LT1028, LT1115, AD797, LME49990||0,9 nV/√Hz|
The second op-amp doesn’t significantly affect the noise specifications, thus its choice is less critical. Some suitable models are listed here.
|Op-amp (second stage)|
|LT1028, LT1115, AD797|
The accuracy of the components used in the two-stage network directly affects the RIAA accuracy. For this reason, resistors are metal film type with a tolerance of 1% or less, and capacitors are metalized film type with a tolerance of 5% or less.
Coupling capacitors (C6, C7, C17 e C18) are metalized film type (any capacitance in the range from 2,2 µF to 10 µF is suitable). Power supply bypass capacitors (from C8 to C11 and from C19 to C22) are ceramic type in order to ensure the minimum impedance at high frequencies.
DIP switches can be set to get 4 different gain values (S2 and S4), 8 values of input capacitance and 4 values of input resistance (S1 and S3).
The two-stage RIAA network
Unlike most RIAA preamplifiers, which use exclusively active or exclusively passive equalization networks, the T-Phonum MKII uses two separate specialized networks simultaneously: an active network is used for low frequencies, while a passive one is used for high frequencies. This technique offers significant advantages, overcoming the limitations of traditional networks.
A typical passive RIAA network introduces an attenuation of about 20 dB at 1 kHz, which needs to be recovered by further amplifying the signal. This places a severe limit on the signal/noise ratio and input dynamics achievable. The two-stage RIAA network avoids this attenuation, therefore maximizing the signal/noise ratio and the input dynamics of the preamplifier. In addition, this approach facilitates the design of low impedance networks without overloading the amplifier stage. This significantly reduces the thermal noise, which is the main problem that affects the traditional high-impedance passive networks.
On the other hand, the use of a passive network only for high frequencies (the 75 µs pole) ensures the proper high-frequency roll-off of the RIAA function even at ultrasonic frequencies, which would not have been possible using an only-active network in the non-inverting configuration. This results in an absolute fidelity in the reproduction of the RIAA function over the entire audio range.
The proper attenuation of the ultrasonic frequencies is also important to prevent intermodulation distortions caused to slew-rate limitations of the following amplifier stages, which thus work in optimal operating conditions.
In summary, the two-stage RIAA network allows benefiting from the advantages of both types of networks while overcoming their limitations. An accurate analysis of the two-stage network, previously presented in this website, has allowed the development of a special algorithm aimed to find the best components values combination to ensure both high accuracy and low noise.
Power supply stage
The T-Phonum MKII board can be powered in many different modes. Therefore, the board can be assembled according to two options:
- the option A, that exploits the power supply stage integrated in the board;
- the option B, that uses an external power supply or a pair of batteries;
A dedicated article illustrates different modalities to externally power the circuit if the option B is used.
Option A (on-board power supply)
The board includes a regulated power supply stage designed to ensure an adequate low noise power supply. In this case, the board is powered by using the E5 connector with a 12 V~ or 15 V~ transformer of at least 6 VA of power, or even with an unregulated dual voltage from ±15 V to ±22 V.
The rectifier consists of four fast diodes in parallel as many capacitors, which ensure a low switching noise. D5 and D6 diodes prevent the output latch of the regulators.
C29, C30 and C31 are ceramic capacitors and they are used for the regulator bypass, whereas C32 compensates for the negative regulator and must be 1 µF tantalum. The indicated typologies of capacitors ensure the stability of the regulators, excellent transient response and low output noise.
Despite the use of an external transformer is the best way to avoid electromagnetic noise, if properly positioned, the transformer can also be mounted inside the enclosure.
Option B (external power supply)
Excluding the integrated power supply stage in the board, it is possible to directly power the op-amps from the E7 connector. This option is useful if a pair of batteries or an external power supply with a dual output voltage suitable for the op-amps is used (usually between ±12 and ±18 V). In this case, components marked with an asterisk (*) shall not be mounted.
Power supply alternatives
The two options allow to power the board: with a single secondary transformer (1), with a center-tapped transformer (2), with a dual unregulated voltage (3) or with a dual regulated voltage (4).
|Opt. A||Mode 1
Single half-wave rectification
|Useful if a single secondary transformer (12 V~ to 15 V~) is used.Uses AC1 and GND pins of E5 connector.|
Full wave rectification
|Useful if a center-tapped transformer (12-0-12 V to 15-0-15 V) is used.
Uses AC1, GND and AC2 pins of E5 connector.
|Useful if a unregulated dual DC voltage (from ±15 V to ±22 V) is used.
Uses AC1 (for positive voltage), GND and AC2 (for negative voltage) of E5 connector.
|Opz. B||Mode 4
Direct op-amp power supply.
|Useful if a regulated dual DC voltage within the absolute maximum rating of the op-amps (usually from ±12 to ±18 V) is used.
Uses V+, GND and V- of E7 connector.
The board shall be mounted in a metal enclosure to ensure adequate shielding. A laser engraved anodized aluminum enclosure is used here. In this case, the ground connection of the enclosure is provided by the power supply connector.
The signal connectors are insulated from the panel and they are connected to the board using RG-174 shielded cable. The turntable ground terminal is connected to the central ground point of the board, where a star connection between the different ground planes of the two audio channels and the power supply is found.
The printed circuit board has been designed to achieve the best noise performance thanks to a careful design of the layout and the ground planes. The dimensions are 10 cm × 12 cm and there are four M3 fixing holes that should be used with insulating spacers. The silkscreen helps the component placing, while a HASL treatment protects the copper and ensures high-quality solders.
Gain and input load settings
The following settings are user selectable.
|Input capacitance and resistance (S1, S3)||1||2||3||4||5|
|Gain (S2, S4)||1||2|
|+40 dB (MM and high output MC)||Off||On|
|+46 dB (MM and high output MC)||On||On|
|+60 dB (only MC)||Off||Off|
|+66 dB (only MC)||On||Off|